I. Field of the Invention:
This invention relates generally to an apparatus and method for assessing effectiveness of changes in therapy on a patient's well-being, and more particularly to the long-term assessment of changes in the mode of pacing of CHF patients on their ability to carry out routine daily activities.
II. Discussion of the Prior Art:
When cardiac pacemakers were first introduced, they were used primarily to pace the heart when the normal conduction path from the heart's sinoatrial node to its atrial ventricular node or from the atrial ventricular node to the ventricles was interrupted. The pacemaker was called upon to deliver ventricular stimulating pulses to maintain a predetermined heart beat rate. More recently, pacing has been used to enhance the hemodynamic performance of the heart in patients suffering from congestive heart failure (CHF) or other type of left ventricular dysfunction. It has been found that by pacing the left ventricle and appropriately setting the AV delay parameter of a pacemaker, the patient's cardiac output can be optimized.
To assess the efficacy of changes made in a pacing parameter such as pacing rate or A-V delay interval, it is of interest to monitor the level of activity of the patient over prolonged periods of time in that it is now well recognized that the activity profile of CHF patients is indicative of their well being and prognosis. That is to say, a direct correlation has been found between a CHF patient's level of activity and their overall well-being. Currently, exercise tests and quality of life questionnaires administered at the physician's office determine the patient's status. It is desirable, however, to determine the status of the patient in a natural setting. Among the many exercise tests that have been employed by physicians in assessing the long-term prognosis of CHF patients has been the "six-minute walk" distance. In a paper entitled "Prediction of Mortality and Morbidity with a Six-Minute Walk Test in Patients with Left Ventricular Dysfunction" by Vera Bittner, M.D., et al. and published in the Oct. 13, 1993 edition of the Journal of the American Medical Association (Vol. 240, No. 14, pp. 1702-1707) a study is described that was performed to determine the potential usefulness of the six-minute walk test as a prognostic indicator in patients with CHF. Based on the distances that the several patients included in the test were able to achieve during a six-minute walk at submaximal effort levels, a definite correlation was found, with the distance walked being inversely related to patient mortality and morbidity. See FIGS. 1 and 2. A similar study with similar outcomes is described in a paper entitled "Using a Six-Minute Walk Test to Predict Outcomes in Patients with Left Ventricular Dysfunction" by Nancy P. Milligan, R. N. et al., published in the July/August 1997 edition of Rehabilitation Nursing (Vol. 22, No. 4, pp. 177-181).
The six-minute walk test is generally carried out under closely controlled conditions in a hospital or clinic. Patients are asked to walk up and down a corridor at their own rate as a medical professional times and calculates the distances traversed in a six-minute period. A pedometer affixed to the patient's leg may be used to determine the distance traversed, but usually the distance is determined from the length of the corridor and the number of times the patient has walked it in a six-minute interval.
Activity can also be measured by means of an accelerometer affixed to the patient. As is pointed out in an article by C.V.C. Bouten et al. entitled a "Triaxial Accelerometer and Portable Data Processing Unit for the Assessment of Daily Physical Activity", published in the IEEE Transactions on Biomedical Engineering, Vol. 44, No. 3, March 1997, pp. 136-147, information derived from a patient-worn accelerometer can reliably be used for the assessment of that patient's physical activity and it offers the advantage that the information may be gathered in the course of more normal patient activities. Studies we have conducted show a direct, positive correlation between the frequency with which the output from a patient-worn accelerometer exceeds a predetermined threshold and the distance traversed during a six-minute walk test protocol. Therefore, we are able to conclude that accelerometer-derived data can be used in assessing patient well-being and we have developed an apparatus and algorithm using an activity sensor to assess the efficacy of a change in pacing therapy on that patient's overall well-being.